The field of the invention is photography with range finder and view finder. The invention is particularly concerned with a mirror reflex or single lens reflex (SLR) camera having a view finder consisting of a focusing screen, a field lens, a pentaprism and an ocular and comprising an electronic rangefinder. In this camera an image of the object to be photographed taken by the objective is projected by a fully specular concave mirror ground on the lower edge of the pentaprism exit face through the pentaprism of the mirror reflex camera.
The state of the art of the present invention may be ascertained by reference to U.S. Pat. Nos. 4,185,191 and 4,188,102 the disclosures of which are incorporated herein.
U.S. Pat. No. 4,188,102 discloses a range finder wherein a concave mirror is ground on the lower edge of the pentaprism exit face and wherein the prism angles are selected so that the view finder axis is directed at an angle downward from the optical axis which in the ordinary use of the camera would be horizontal. Moreover, with respect to its horizontal optical axis, in the ordinary use the ocular is upwardly offset and sloped so that an angle is subtended between the ocular and view finder axis.
Due to the tilt of the optical axis for the viewer optics on the one hand and the measuring beam on the other one, it is possible that the changes in angle required in the pentaprism result in undesired reflections in the viewer image. Furthermore, the mutual tilting of these two optical axes results in a bothersome darkening of the viewer image when low-light, long-focal length objectives are used. The exit pupil in these cameras is well known to be smaller than the ocular window.
U.S. Pat. No. 4,185,191 discloses particularly in FIGS. 10,11 and 14 an electrical range finder wherein an image of the object to be photographed is projected onto the image plane of the picture-taking objective or onto a single plane conjugate to it, wherein on an array of photoelectric detectors is mounted. The detectors are preceded by a plurality of lens elements so arranged that two adjoining detector elements each receive the light from one half-pupil of the picture-taking objective only. It is difficult to make such a device because the size of the pupil images varies with the relative aperture of the objective. Therefore, a delicate adjustment is required especially when objectives with small relative apertures are used, because the light beams from the two objective halves become different even by small lateral offsets. With small stops this system therefore becomes unusable. On the contrary using large relative apertures light beams from given pupil halves may reach neighboring detector elements, which again would be source of error.